The present invention relates to a stainless steel which has a graphite thin film containing carbon existing in a state of diamond (hereinafter described as "diamond carbon"), suitable for use in decorating purpose and for use as corrosion-resisting materials such as architectural materials and automobile materials, and also to a method of producing such a stainless steel material by making use of chemical vapor deposition.
In general, stainless steels have wide use as architectural facings and interior finishes because of their attractive appearance. In recent years, there is an increasing demand not only for stainless steel materials having native surfaces but also for stainless steel materials having specific colors for decorating purposes.
Hitherto, various wet coloring methods have been proposed for coloring stainless steel, such as so-called INCO coloring method. Stainless steel materials processed by such wet-type methods, however, tend to exhibit color fading, discoloration and decoloration, as well as deterioration in weatherability, during long use. This drawback seems to be attributable to the presence of moisture content included during the coloring process. In addition, the stainless steels processed by this type of method are liable to be scratched easily, thus exhibiting inferior wear resistance.
Recently, it has been desired to obtain variation regarding each color, for example, to obtain a deep black color tone having transparent feeling. Although it became possible to obtain black color appearance by improving a prior art method, it is a single usual black color, that is, it had not been able to obtain various black colors or to obtain various black color surfaces having improved weatherability and wear-resisting property.
Technology for synthesizing bulky diamonds under high pressure and temperature has been developed and put into use in industrial scale in the 1960's . Technology for synthesizing thin diamond film, however, is rather new as compared with the synthesis of bulky diamonds. The synthesis of thin diamond film, which pertains to synthesis of diamonds in graphite stable region, belongs to a very interesting field from an academic point of view. In addition, synthesized thin diamond film has a potential for allowing various advantageous features of diamond t be utilized in various technological and industrial fields. Nowadays, studies are made for developing processes for synthesizing diamond thin films in various fields such as material industries, e.g., coating of cutting tools, semiconductor industries, e.g., semiconductor devices and heat-dissipating substrates of semiconductor devices, acoustic technologies, e.g., diaphragms of speakers, and optical technologies. Attempts and proposals for utilization of diamond thin films are disclosed, for example, in Japanese Unexamined Patent Publication No. 232991/1984.
Actually, however, such attempts and proposals are successfully put into practical use only in limited fields such as partial coating of tool materials and acoustic engineering in which a small area is coated, because of difficulty encountered in forming thin films of diamond. It is to be pointed out also that these attempts and proposals are intended only for the production of thin film having 100% diamond content, and there are very few attempts intended for forming thin films to which the invention pertains, i.e., films containing both diamond carbon and graphite, as shown in Japanese Unexamined Patent Publication No. 65796/1985. In the prior art of the Japanese Unexamined Patent Publication No. 65796/1985 there is disclosed a hard carbon film comprising diamond of hexagonal system and graphite, however, in the Publication there is disclosed no teaching regarding the ratio of diamond to graphite. Further, in the Japanese Unexamined Patent Publication No. 65796/1985, the hard carbon film is intended to be used as a sliding part mounted in machine or as cutting tools, and the thin film is described to have high hardness, wear-resisting property and lubrication property, however, there is no description regarding decoration property and more particularly regarding color. In this Japanese Publication, regarding a method of producing the thin film, there is disclosed only a method of chemical vapor deposition by using D.C. plasma of thermionic emission type, in which method there has been such a drawback that a good film is obtainable regarding only a small area in a case of using the chemical vapor deposition using D.C. plasma of the thermionic emission type. In addition, there is no disclosure regarding the ratio of diamond to graphite in this Japanese Publication.
Further, it has also been proposed to make use of amorphous carbon thin film for decorating purposes, as disclosed in U.S. Pat. No. 4,524.106. The art shown in this Patent, however, cannot provide transparent black color tone, although it provides a single color approximating pure black. In addition, the single black color film formed by this art is still unsatisfactory in that it has only insufficient hardness, although it exhibits superior weatherability and, therefore, this art has not been put into practical use.
In the prior art, the coating of a sliding part used in a machine or of a cutting tool with diamond carbon has high hardness and wear resistance. However, when using the coating as the surface of a decorative material, it has been desired to have a stainless steel with various color tones even in a case of a black color and more particularly black color having both transparent feeling and various degree of the transparent feeling, instead of conventional single black color tone. In addition, it has been desired to obtain a method of producing a diamond carbon film of a large area and of making it possible to control the ratio of generated diamond carbon and graphite.
The present invention makes it possible to simultaneously and quantitatively form a film of a mixture of graphite and diamond carbon on a surface of stainless steel to thereby obtain a black transparent film which has not hitherto existed and to thereby make the degree of the transparency variable to a desired level. Further, since the film has a hardness value approximating to that of diamond, the film has a superior resistance to being damaged.